The present invention relates to safety systems to shut off machinery when a door of a machinery enclosure is opened, and more particularly to a pneumatic safety system.
At the present time there is an increased emphasis on enclosing machines within a noise-reduction enclosure to aid the health and safety of workers within a factory. Regulations issued under the United States Occupational Health and Safety Act, known as OSHA, have set standards regarding the noise levels within areas of a factory. It may be either costly or impossible in many cases to redesign machines, such as presses, so that the machines will meet the relatively low noise levels required by OSHA.
One suggested solution has been to enclose the noisy machines within a noise-reducing enclosure. For example, U.S. Pat. No. 3,860,086, issued Jan. 14, 1975 to Mahajan et al, discloses an enclosure housing comprised of removable panels. That patent points out that many machines require ready access and attention to ensure smooth operation. However, that patent does not disclose any safety measure by which removal of one of the panels will cause the machine to shut down in order to protect the worker during maintenance or repair operations.
At the present time there is a continued interest in improving equipment to protect the safety of personnel who inspect or service the machinery, which is placed in a noise-reduction enclosure. Those machines must still be serviced, for example, upon malfunction, for cleaning, inspection, repairs, oiling, and other maintenance. If the enclosure door is opened while the machine is still operating, the employee may possibly be injured.
A series of safety devices, sometimes called "interlock devices," have been suggested to automatically shut off a machine when a door to its enclosure is opened. However, certain of these interlock devices present various difficulties in operation. For example, if the interlock device is a series electrical circuit having switch contacts at the enclosure door, the switch or other circuit component may fail and the relay which shuts off the machine may fail to be operated. In addition, repair personnel may readily defeat an electrical safety system by jumping the switch with wires. Although the safety interlock system may have been installed to protect the health of the employees, it has been found that they may short-circuit or otherwise defeat a safety system for what they believe is their convenience.
As another example, a safety interlock system has been suggested in which magnetically operated electrical reed switches in a door frame are closed by magnets in the door. However, that reed relay system is easily defeated by placing a loose magnet on the reed switch when the door is opened.
The present invention utilizes a pneumatic safety system for the doors of an enclosure. The use of pneumatic safety or alarm systems has been suggested in prior art patents. U.S. Pat. No. 583,405, entitled "System of Pneumatic Doors and Gratings," issued May 25, 1897, describes a pneumatic system for prisons and vaults in which a door frame member is part of an air valve, described as a "valve joint." When the door is opened air from a reservoir escapes through the opened valve at the door and sets off an electric alarm. The "valve joint" of the 583,405 patent is limited to a hinged door and is not applicable to a sliding panel or a sliding door. In addition, the system may be defeated by inserting the round nozzle of an air hose into the round port "g" of the air pipe.
U.S. Pat. No. 3,921,563, entitled "Pneumatic Alarm System" and issued Nov. 25, 1975, describes a fire and burglar alarm system using a pressurized gas container connected to a gas line. The gas line is connected to rupturable sensors which, when broken by fire or opening a door or a window, permit release of the gas, setting off an alarm. The sensors are "one-shot," i.e., they must be replaced after they are ruptured. The system is not applicable to an enclosure in which the doors may be opened many times a day and where each time the door is opened it is desired to shut off the machine within the enclosure.